Every program running on the command line has three data streams connected to it.

• STDIN (0) - Standard input (keyboard for example)
• STDOUT (1) - Standard output (defaults to the terminal)
• STDERR (2) - Standard error (for error messages, defaults to the terminal)

The program usually gets its arguments from STDIN and outputs messages to STDOUT. The error messages will be output to STDERR. Redirection and pipe can be used to change the source of the argument or the destination of the output message.

STDOUT Redirection

STDOUT redirection can be used to save the output of a program to a file instead of STDOUT (printing it on the terminal). The basic usage is

fool@bar:~$ command > result.txt

The output message of command will be save to result.txt. It should be noted result.txt will be created if it doesn't exist and its content will be erased if it is not empty. >> can be used if the content of result.txt needs to be kept.

STDERR Redirection

STDERR redirection can be used to save the error message of a program to a file instead of STDERR. The basic usage is

fool@bar:~$ command 2> error.log

The error message of command will be save to error.log.

STDIN Redirection

STDIN redirection can be used to send the program some arguments. The basic usage is

fool@bar:~$ command < parameter.txt

The content of command will be sent to command.

Pipe

pipe takes the STDOUT of a command and pass it to the STDIN of the next command. The basic usuage is

fool@bar:~$ command 1 | command 2

Here are some useful case of pipe.

• count number of files

fool@bar:~$ ls | wc -l

• obtain the 5th and 6th file.

fool@bar:~$ ls | head -n6 | tail -n2

• find the keyword

fool@bar:~$ cat file | grep "keyword"

• sort and find the unique value

fool@bar:~$ cat file | sort | uniq